You want to prevent the transition from a GPL codebase with some public domain code to a public domain codebase with some GPL code. One way to do so is to outright ban contributions leveraging tools that are able to generate public domain code at superhuman speeds.
As you pointed out yourself, there's always the option to create a fork that does allow AI contributions, which may eventually force a re-assessment of the policy if the gap in utility grows too large.
As you pointed out yourself, there's always the option to create a fork that does allow AI contributions, which may eventually force a re-assessment of the policy if the gap in utility grows too large.